ChemHistory
Chemistry
of the Lightbulb
Still a Bright Idea
By Brian Rohrig
With the flick of a switch, we are instantly bathed in visible light.
When the power goes out and we scramble for candles and a
flashlight, we realize how much we rely on the electric lightbulb.
Few inventions have changed our lives as much as this remarkably
simple, yet ingenious device.
PHOTO ABOVE: PHOTODISC
Edison’s
invention
Although Thomas Edison was
not the first person to patent an electric lightbulb, he made so many
improvements on its design that history generally gives him the credit.
Without Edison’s improvements,
lightbulbs would last about as long as
candles. In 1879, he constructed a
lightbulb that glowed continuously
Thomas Edison
Lewis Howard Latimer
These first lightbulbs were
incandescent, as are most of the
lightbulbs in your home today. Incandescence is the property of giving off
visible light when heated. The hotter
the object, the more energetic the
light that is given off.
As you have probably observed,
a piece of metal glows if its temperature gets high enough. As the temperature increases, the color of the
emitted light changes from dull red,
to orange, and at about 5800 °C, to
white. A typical incandescent lightbulb operates at a temperature of
about 2500 °C, where it glows with a
yellow-white light.
Finding just the right filament was not an easy task.
Using a simple trial-and-error
approach, Edison experimented
with thousands of different
types of filaments. He was looking for one that would be both
long-lasting and affordable.
© NEW YORK PUBLIC LIBRARY
for 40 hours, but he was determined
to do better than that. By the end of
1880, his 1500-hour lightbulb was
ready for public sale. A mere 25
years later, his electric bulb had forever transformed human life by illuminating homes and cities all over
the world.
© NATIONAL PARK SERVICE
T
he fact is, we rely on the electric lightbulb. Few inventions
have changed our lives as
much as this deceptively simple, yet
ingenious device.
With the flick of a switch, we are
instantly bathed in visible light. When
the power goes out, we scramble for
candles and matches. But the charm
of glowing firelight soon wears off,
especially when sports events or
homework are in the plans.
ChemMatters, APRIL 2003 11
ALL PHOTOTS MIKE CIESIELSKI
Argon to the rescue
Although we talk about glowing lightbulbs, the
actual glow comes from only one bulb
component—a very thin wire called the filament.
A typical 60-watt bulb contains about 2 meters of
very thin tungsten wire only about 25 micrometers (1/1000 inch) thick. Look very closely at a
clear unlit bulb, and you’ll see that the filament
is tightly wound into a double coil.
The first breakthrough came when Lewis
Howard Latimer, the only African-American
member of the Edison research team, developed an improved carbon-based filament that
yielded an extended glow. He went on to
design efficient production methods for manufacturing them in commercial quantities at
reasonable cost.
Cost was a significant consideration for
the lightbulb to catch on with the public. For
example, Edison’s team experimented with
long-lasting platinum but rejected it for its
price tag.
In 1910, William Coolidge of the General
Electric Company developed a tungsten filament that is still in use today. Tungsten metal,
with a melting point of about 3680 °C, proved
to be both affordable and long-lasting.
How does a
lightbulb light?
When electric current follows the metal
pathway through an incandescent lightbulb, a
tremendous amount of resistance is encountered as the electrons make their way through
the very thin wire filament. Similar to students
in a crowded lunch line, closely packed electrons generate friction—and that, in turn, produces heat. The thinner the wire, the more
resistance the electrons encounter as they
attempt to pass through. More resistance generates more heat until the wire reaches a high
enough temperature to produce visible light.
Lightbulbs “burn out” when the filament
breaks. At high temperatures, the tungsten
metal, like other metals, sublimes. Sublimation is a process by which a solid is converted
directly into a gas without first passing
through a liquid phase—like old trays of ice
cubes shrinking in your freezer.
12 ChemMatters, APRIL 2003
In lightbulbs, the sublimated tungsten
atoms are still sealed inside with nowhere else
to go. Examine a burned-out lightbulb and
you’ll see a tungsten deposit on the interior
surface of the glass—clearly visible as a black
spot on top of the bulb. Even with its low sublimation rate at high temperatures, the tungsten filament finally wears thin and breaks. As
a result, the circuit is broken. When the metal
pathway breaks, electrons stop flowing, and
the lightbulb ceases to glow.
Finding the right filament was only one
hurdle for the early inventors. The second big
problem came from the surrounding air—
specifically oxygen.
We talk about “burning” lightbulbs, but
the fact is that the filaments inside successful
bulbs glow without burning. Edison found that
for many filaments, the presence of oxygen
caused the hot material to rapidly combust
and break. Since glowing doesn’t require any
oxygen, Edison’s early lightbulbs consisted of
filaments mounted in a vacuum.
Although the vacuum solved one problem, it created another. Without any gas exerting pressure on the filament, the rate of
sublimation increased. The tungsten atoms of
the solid filament readily entered the gas
phase at the high temperatures within the
glowing bulb.
www.chemistry.org/education/chemmatters.html
With the discovery of the noble gas
argon in 1894, it became possible to lengthen
the life of a lightbulb by filling it with this very
unreactive gas. Not only did argon offer an
oxygen-free filler, it also controlled the sublimation rate of the filament by transferring
some of the excess heat away from the glowing metal.
The transfer process is called convection. As atoms of argon bump into the hot filament, some of the kinetic energy of the
tungsten atoms is transferred to the argon
atoms. This transfer cools the filament and
heats the argon. The argon atoms then speed
off to collide with the inner wall of the glass
bulb. Upon impact, the argon atoms transfer
some of their kinetic energy to the glass, raising its temperature.
In the process, glowing incandescent light
bulbs become blistering hot. In fact, about 90%
of the electrical energy consumed by an incandescent lightbulb is dissipated
as heat.
Krypton would be a better noble gas to use in a
lightbulb than argon, since
it is a poorer conductor of
heat. But krypton is
very expensive.
The only place
to get krypton
is from the
atmosphere,
where its concentration is only about 1 part
per million (ppm). By contrast, argon comprises nearly
1% of the atmosphere, making it 10,000 times more
abundant than krypton.
Krypton is usually used to fill small flashlight
bulbs. Because argon-filled bulbs readily transfer
energy away from the glowing filament to the
glass, batteries drain rapidly in the process. But
when poorer-heat-conducting krypton fills the
space, the bulb feels cool to the touch even after
extended use, and batteries last longer.
Extended-life bulb
An extended-life lightbulb contains a
much longer filament. As a result, there is
more surface area to dissipate the heat. The
filament does not get as hot and does not
sublimate as quickly.
The drawback is that the light produced
by the extended-life bulb is dimmer and redder. It takes a higher wattage to produce the
Heat lamp bulb
A heat lamp is essentially an incandescent bulb with a very long filament. The filament has a long life and glows a dull red
color. The cooler filament emits most of its
energy as longer-wavelength infrared light,
which cannot be seen but will still heat up
objects on which it falls. The globe of the bulb
is very large; this increased surface area
allows it to radiate MIKE CIESIELSKI
more heat. This
design makes the
heat lamp very
practical, since its
intended purpose
is to give off heat,
not light.
Fluorescent lamps
Since all incandescent bulbs give off a
great deal of heat, cool fluorescent lamps offer
a much more efficient alternative. Introduced
in the 1950s, they soon became widely
accepted for nearly all schools, offices, and
commercial buildings.
Fluorescent lamps consist of a sealed
glass tube containing a mixture of noble gases
and a few drops of mercury that vaporize
within the tube. When an electric current
passes through the gas in the tube, some of
the electrons of mercury become excited.
Excitation occurs when electrons absorb
energy and temporarily achieve a higher
energy level. As they return to ground state,
the energy previously absorbed by the electrons is primarily released as ultraviolet (UV)
light, a light with shorter wavelength and
greater energy per photon than visible light.
Because UV light is invisible to humans,
the fluorescent lamp must convert it into visible light. This is accomplished by the white
phosphor coating on the inside of the bulb.
When UV light strikes this phosphor coating, it
is converted into visible light. Thus, unlike
incandescent bulbs, fluorescent lamps do not
give off light by heating any of their components. That makes them much more energyefficient.
If you have a UV-sanitizing cabinet for
TRY THIS!
an you assemble these three familiar items correctly so that the light
bulb lights? You’ll need a 12-inch piece of insulated wire with exposed
metal at each end, a C or D-cell power source, and a flashlight bulb.
If you succeed on the first try, you are a shining example to us all! A wellknown video distributed to educators by the Private Universe Project in 1989
shows recent Harvard and Massachusetts Institute of Technology (MIT) grads
struggling with the challenge. In the video, one frustrated individual exclaims,
“I’m a mechanical engineer, not an electrical engineer!” (Private Universe Project, A Private Universe [Videotape],
Harvard-Smithsonian Center for Astrophysics: Cambridge, MA, 1989)
C
MIKE CIESIELSKI
same amount of light as a typical incandescent bulb. Wattage refers to how much energy
is used per second. So, even though they last
longer, extended-life bulbs may not really be a
bargain. They actually cost more money due
to the greater wattage required to operate
them.
The answer appears on page 20.
goggles in your classroom, examine the bulb
inside. It will resemble a typical fluorescent
bulb, but with one big difference. It’s transparent. The lack of a phosphor coating means
that the bulb emits UV light, with very little
visible light. It is this UV light that kills
microorganisms and sterilizes your goggles.
descent lightbulb. Generating intense heat,
halogen lamps have been known to cause
fires. Homeowners must take care to keep the
lamps away from draperies and other combustibles. Furthermore, since halogen bulbs
give off so much heat, their energy efficiency
gets low marks.
Halogen bulb
Neon lights
Halogen bulbs, another type of incandescent bulb, produce intense white light.
They are commonly used in car headlights, floodlights, and other applications where very bright light is needed.
Halogen bulbs, as their name implies,
contain the vapor of a halogen (group
17 on the periodic table), usually
bromine or iodine. The halogen molecules
act as chemical scavengers, picking up stray
tungsten atoms that have sublimed and
depositing them back on the filament. The
unique ability of the halogen atoms to combine with tungsten atoms means you’re not
likely to find black spots on the inside of the
bulb. However, the filament of the bulb eventually breaks due to uneven deposition of
tungsten atoms on the filament.
Since the filament lasts much longer,
halogen bulbs are designed to glow several
hundred degrees hotter than a typical incan-
TO
P HO
C
DIS
Neon signs are similar to
fluorescent lamps, except
that they contain no mercury or phosphor coatings. A mixture of neon
and other gases within
the tube gives off colored
light when the electrons are
excited by an electric current.
Light-emitting diodes
Eventually, both incandescent and fluorescent bulbs may give way to light-emitting
diodes (LEDs). The indicator lights on computers and the numbers on digital alarm
clocks utilize LEDs—light sources based on
the properties of semiconductors such as silicon. For an explanation of how these durable
devices operate, see “Light-Emitting Diodes—
Tune in to the Blues” in the April 2001 issue of
ChemMatters.
Brian Rohrig teaches chemistry at the Eastmoor Academy in Columbus, OH. His most recent article for
ChemMatters, “Matches—Striking Chemistry at Your Fingertips”, appeared in the December 2002 issue.
REFERENCES
Bloomfield, Louis A. How Things Work: The Physics of Everyday Life; John Wiley & Sons: New
York, 1997.
Sacks, Oliver. Uncle Tungsten: Memories of a Chemical Boyhood; Alfred A. Knopf: New York,
2001.
ChemMatters, APRIL 2003 13